Digital-to-analog converters (DACs) play a critical role in transforming information from a digital domain into an analog domain. DACs generally convert an abstract finite precision number (such as a fixed-point binary number) into a physical quantity (such as a voltage or current). The number of output levels for a given DAC generally corresponds to its resolution, which is based on the number of “bits” of a given digital code that defines each output level. A variety of DAC architectures exist, including voltage-mode schemes and current steering schemes.
FIG. 1 illustrates a voltage-mode DAC, generally designated 100. The DAC includes a data input 102 that receives a data word of multiple bits during a bit time and a clock input 104 that receives a clock. Respective supply (VDD) and ground (VSS) reference voltages are provided to the DAC. Switch circuitry (not shown) responds to the clock to generate corresponding outputs that are weighted and summed to create an overall output voltage level corresponding to the input word code, but in the analog domain. An important consideration in achieving successful operation of the DAC involves maintaining an aggregate matching characteristic impedance for the output of the DAC, such as, for example 50 ohms.
FIG. 2 illustrates one conventional approach in tuning a DAC output impedance to exhibit an aggregate output impedance of 50 ohms for a DAC. The DAC includes respective DAC drivers 2000-200n-1 for processing each “bit” b0-bn-1 of the input N-bit word. Each driver, or bit slice, includes an output impedance unit 202 that exhibits an output resistance ROUT. Switch circuitry 204 selectively couples the impedance to the output “output” based on the value of the received input bit. Since multiple drivers are employed, sample-to-sample variations may cause the actual resistance of each output impedance unit to vary.
One solution to providing a post-silicon tuning method, illustrated in FIG. 3, involves configuring each output impedance unit as multiple separately selectable impedance units in series, such as shown in 302. A large impedance of, for example, 14 kohms, may be activated by switch 304, and tuned by selectively switching in one or more smaller impedance units of, for example, 1 kohms each to approach 16 kohms per slice. Aggregating the output impedances from the other bit-slices in parallel results in an aggregate output impedance value close to the desired 50 ohms. While this may appear to address the problem of inaccurate output impedance, it may introduce additional capacitance to the output node which may alter the termination impedance at higher frequencies.
What is needed is an accurate and power-efficient method and apparatus to support output impedance tuning operations for a voltage-mode DAC driver.